Nuclear power reactors are well known and are discussed, for example, by M. M. El-Wakil in "Nuclear Power Engineering" McGraw-Hill Book Company Inc., 1976.
In a known type of nuclear power reactor, for example, as used in the Dresden I reactor of the Dresden Nuclear Power Station near Chicago, Ill., the reactor core is of the heterogenous type. In such reactors the nuclear fuel comprises elongated rods formed of sealed cladding tubes of suitable material, such as a zirconium alloy, containing uranium oxide and/or plutonium oxide as the nuclear fuel, for example, as shown in U.S. Pat. No. 3,365,371. A number of such fuel rods are grouped together and contained in an open-ended tubular flow channel to form a separately removable fuel assembly or bundle. A sufficient number of fuel assemblies are arranged in a matrix, approximating a right circular cylinder, to form the nuclear reactor core capable of self-sustained fission reaction. The core is submerged in a fluid, such as light water, which serves both as a coolant and as a neutron moderator.
A typical fuel assembly is formed by an array of spaced fuel rods supported between upper and lower tie plates, the rods being several feet in length, on the order of one-half inch in diameter and spaced from one another by a fraction of an inch. To provide proper coolant flow past the fuel rods it is important to maintain the rods in spaced position and restrain them from bowing and vibrating during reactor operation. A plurality of fuel rod spacers spaced along the length of the fuel assembly are provided for this purpose. Such spacers are shown, for example, in U.S. Pat. No. 3,654,077.
As positioned in the reactor core, the fuel assemblies are spaced from one another and thus are surrounded by a slab of the water-moderator. Thus the peripheral fuel rods are exposed to a higher thermal neutron flux than the fuel rods of the inner and less moderated region of the fuel assembly.
Flatter thermal neutron flux across the fuel assembly, with improved fuel utilization and decreased local power peaking, can be achieved by increasing the neutron moderation in the inner portion of the fuel assembly. This may be accomplished by replacing one or more of the inner fuel rods with water tubes formed with passages by which relatively cool water is conducted upwardly through the inner portion of the assembly. An arrangement of such water tubes is shown in U.S. Pat. No. 3,802,995.
Moderation through the inner portion of the fuel assembly can be increased by increasing the number of water tubes. However, this approach has a practical limit since each water tube replaces a fuel rod and thus undesirably decreases the fuel containing capacity of the fuel assembly.
An object of this invention is to increase the neutron moderation in the inner portion of a fuel assembly without increasing the number of water tubes. Another object is a fuel rod spacer with spring means for accommodating water tubes of larger diameter than the fuel rods.